In a nuclear power plant, a failure in the means intended for the removal of residual power from the nuclear reactor may induce a loss in cooling of the nuclear fuel. In some circumstances, this loss may lead to partial or total core meltdown. The probability of such an accident, although extremely small, is not zero.
In order to prevent and manage the consequences of such an accident both on the population and the environment, a severe accident in a nuclear reactor core (in particular a Pressurized Water Reactor (PWR)) is simulated by melting sections of nuclear fuel rods that have previously been irradiated, in an induction furnace made of oxide-based refractory materials.
During such experiments, in which the temperature is varied (reaching up to 2600° C.) and the atmosphere is changed (for example, to neutral or oxidizing), the nuclear fuel behavior is studied, and the fission products and actinides which are released are identified and analyzed.
The oxide-based refractory parts of the furnace must at least fulfill the following criteria:                mechanical strength up to a temperature of 2600° C.,        tightness to gases generated during the experiment,        chemical resistance to various types of atmosphere (in particular reducing, oxidizing, neutral, comprising air, water vapor),        chemical resistance to corrosion and/or high temperature ablation, which may be caused by baths consisting of oxides and metals, for a duration of at least 15 minutes. Such interactions are generated, for example, when the refractory material is brought into contact with the corium. The latter is magma which results from the high-temperature meltdown of the nuclear fuel, followed by a reaction of the molten fuel with its cladding and the structural elements of the nuclear reactor. Most often, it is composed of corrosive baths of oxides and metals, heated-up above their melting point or solidus temperature. These oxides are, in particular, uranium, zirconium and iron oxides.        
In order to fulfill such criteria, until 2003, the refractory parts of these furnaces were manufactured from thorium dioxide ThO2, which oxide has a melting temperature of 3380° C.
However, since thorium dioxide ThO2 is radioactive, it is difficult to implement and attempts are being made to replace it by another non-radioactive refractory material which also fulfills the above criteria. One candidate material is hafnium dioxide HfO2.
Hafnium dioxide HfO2 has three crystalline structures, each of which has its own stability domain as a function of temperature and pressure. At atmospheric pressure, these domains are as follows:                below 1700° C.: monoclinic structure,        from 1700° C. to 2600° C.: quadratic structure,        from 2600° C. to 2810° C.: cubic structure,        above 2810° C.: liquid state.        
Hafnium dioxide HfO2 has a melting point of 2810° C. and is also well known to withstand chemical interactions under heat. It therefore appears to be a good candidate for the replacement of thorium dioxide ThO2 as a refractory material used in the composition of furnace parts enabling the above-mentioned simulations to be carried out.
However, pure hafnium dioxide HfO2 has a major drawback in high temperature applications, in that, during thermal cycling (increasing/decreasing temperature), its allotropic transformation from the monoclinic phase to the quadratic phase is accompanied by a 3.4% shrinkage (or a volume expansion during the opposite transformation) between 1500° C. and 1800° C. As an unacceptable consequence of this large volume change, cracking of the refractory ceramic material consisting of hafnium dioxide HfO2 occurs.
From U.S. Pat. No. 5,681,784, it is known that this volume change may be prevented by stabilizing the hafnium dioxide HfO2 in its cubic phase (high temperature phase) by means of additives. For that purpose, to the hafnium dioxide HfO2 material, 8 mol % to 12 mol % yttrium oxide Y2O3 and 0.25% to 4% by weight of sintering aids, are added. The material thus obtained has a low solidus temperature (well below 2500° C.), which makes it inappropriate for use as a constituent refractory material in the above-mentioned simulation furnaces.